Obama and Homeland Security Secretary Jeh Johnson, who administered the oath of citizenship, presided at a ceremony at the National Archives. Joanne Ferreira, a spokeswoman for U.S. Citizenship and Immigration Services, said the 50 new citizens were selected at random from candidates living in the Washington area and are from 44 countries.
The first lady said that there is near-universal agreement the immigration system is broken -- but still debate on whether to fix it. She said her husband has that as a primary goal.
"He refuses to give up the fight, because at the end of the day this fight isn't about abstract principles; it's about real people," Obama said. "People like you. People like us. Our fellow Americans."
The first lady suggested that the United States should welcome newcomers.
"Immigration is at the heart of how we developed as a nation," she said. "We still, very much, are a nation of immigrants."
About 600,000 to 700,000 people become naturalized U.S. citizens every year. Millions of people are believed to be living in the country illegally.